Nanoscale doping heterogeneity in few-layer WSe_2 exfoliated onto noble metals revealed by correlated SPM and TERS imaging

Abstract

While extensive research effort has been devoted to the study of the 2D semiconductor–insulator interfaces in transition metal dichalcogenides (TMDCs), there is little knowledge about the electronic quality of the semiconductor–metal interface in the atomically thin limit. Here, we present the first correlated nanoscale mapping of the interface of atomically thin WSe_2 with noble metals using co-localized scanning probe microscopy and tip-enhanced optical spectroscopy (TEOS), such as tip-enhanced Raman spectroscopy (TERS). Nanoscale maps of the topography, surface potential, Raman spectra, and the photocurrent amplitude of the WSe_2/metal interfaces reveal striking results. Specifically, correlations between surface potential, resonant Raman signatures and photocurrents that indicate the presence of inhomogeneities within interfacial electronic properties, which we attribute to variations in the local doping of the WSe_2 likely caused by intrinsic compositional fluctuations or defects. Our results suggest that local electrostatic variations at a lateral scale of 10–100 nm are present even in the highest quality of TMDC crystals and must be considered towards understanding of all interfacial phenomena, particularly in device applications that rely on the buried metal–semiconductor junction interface.